Thermal inkjet hardcopy devices such as printers, graphics plotters, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle and has a firing resistor located in the chamber. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses energize the ejection element, a drop of ink is ejected from the printhead. Properly arranged nozzles form a dot matrix pattern.
The ink cartridge containing the ink ejection elements is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the resistors is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as high as the number of ink ejection elements arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the height of the swath or a fraction thereof, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
The print quality produced from an inkjet device is dependent upon the reliability of its ink ejection elements. A multi-pass print mode can partially mitigate the impact of the malfunctioning ink ejection elements on the print quality by substituting functioning ink ejection elements for malfunctioning ink ejection elements. This is possible in a multi-pass printmode because more than one ink ejection element traverses each horizontal print position, or row, on the media. For example, in a two-pass printmode two ink ejection elements pass over each horizontal print position on the media and in a four-pass printmode four ink ejection elements pass over each horizontal print position on the media. Thus, in a two-pass printmode one other functioning ink ejection element may be substituted for a malfunctioning ink ejection element and in a four-pass printmode three other ink ejection elements may be substituted for a malfunctioning ink ejection element. However, use of multi-pass printmodes significantly reduce printer throughput.
However, when printing in a one-pass printmode the ability to hide a malfunctioning ink ejection element with a different ink ejection element is not possible because all pixels in a horizontal row are always printed with the same ink ejection element. If this malfunctioning ink ejection element is out there is no way to hide the malfunctioning ink ejection element with error hiding techniques that depend on multiple passes. Accordingly, a method is needed which corrects for malfunctioning, or inoperable, ink ejection elements in a one-pass printmode.
A method of correcting for malfunctioning ink ejection elements in a printing system using a single pass over a recording medium which includes obtaining a standard printmask, identifying ink ejection elements which are malfunctioning, ascertaining an original color measurement value for each pixel which will be printed with the malfunctioning ink ejection elements, determining a replacement color measurement value closest in value to the original color measurement value which does not use the malfunctioning ink ejection elements for each pixel which will be printed with the malfunctioning ink ejection elements and modifying the standard printmask by adjusting the number and color of ink drops deposited based on the replacement color measurement value for each pixel which will be printed with the malfunctioning ink ejection elements to create a modified printmask.